Robert Haas <robertmhaas@gmail.com> writes:
> On Wed, Apr 8, 2009 at 1:17 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> And please note that we think bitmap scans are the larger part of
>> the win anyway. �What's left undone there is some marginal mopup.
> Can you elaborate on this? I'm fuzzy on why index scans can't benefit
> from this as much as bitmap index scans.
The main point is that the planner will prefer a bitmap scan for any
query that's estimated to return more than quite a small number of rows.
(In my experience the cutover point is in the single digits.) So
there's just not that much room to win for plain indexscans. Their
principal application is really for fetching single rows, a case where
prefetch is entirely useless because you have nothing to overlap.
regards, tom lane